1. Field of the Invention
The present invention pertains to remote sensing through three-dimensional imagery and, more particularly, to three-dimensional remote sensing from a flying platform.
2. Description of the Related Art
A significant need in many contexts is to locate and determine the position of things relative to some point. For instance, in a military context, it may be desirable to determine the position, or to locate an object relative to a reconnaissance or weapons system so that the object may be targeted. Radio detection and ranging (“RADAR”) systems are popularly known for use in remotely sensing the relative position of incoming enemy aircraft. RADAR uses radio frequency (“RF”) electromagnetic waves to detect and locate objects at great distances even in bad weather or in total darkness. More particularly, a RADAR system broadcasts RF waves into a field of view (“FOV”), and objects in the FOV reflect RF waves back to the RADAR system. The characteristics of the reflected waves (i.e., amplitude, phase, etc.) can then be interpreted to determine the position of the object that reflected the RF wave.
Some RADAR systems employ a technique known as “Doppler beam sharpening” (“DBS”). DBS uses the motion of an airborne RADAR to induce different Doppler shifted reflections from different cells on the ground. For a fixed range the cells have different Doppler frequencies because each is at a different angle relative to the source of the RADAR wave. This angle comprises depression and azimuth components in rectangular coordinates or, in polar coordinates, a “look angle.” Thus, projections of the RADAR's velocity on each cell differ, thereby allowing for discrimination of each from the other. Azimuth resolution comes from the Doppler frequency, while range is retrieved from travel time. Azimuth resolution is related to Doppler filter bandwidth which is inversely related to the integration time of that filter—the aperture time.
However, DBS RADAR systems have range dependent resolution and a blind zone dead ahead of the DBS RADAR's motion. The blind zone results because, ahead of the platform, there are insufficient differences in the Doppler shift generated by the cells for the DBS RADAR system to distinguish them. More technically, DBS RADARs have problems pulling cells out of fields of view directly ahead of flight because, for a given resolution, any separation between adjacent iso-Doppler curves becomes too narrow. That is, the iso-Doppler contours get too close together for a fixed resolution and to resolve them requires ever-narrower filters compared to broadside ground-cells.
Reflections also present what are known as “Doppler ambiguities” in the field of view where the field of view encompasses both sides of the boresight. The ambiguities arise because cells close to the boresight and the same distance off the boresight will have the same Doppler returns. That is, returns from cells equidistant from the boresight are indistinguishable. This causes ambiguities during processing because it cannot be determined from which side of the boresight a return came.
Some remote sensing systems employ optical signals, e.g., laser detection and ranging (“LADAR”) systems. LADARs send out optical signals that are reflected and from which the reflecting object's position, and sometimes identity, can be determined in a manner analogous to RADAR. LADARs with both a transmitter and receiver, or SALs (semi-active laser seeker) with only a receiver, encounter problems that arise from the fact that they use traditional optical technology. For instance, airborne LADAR and SAL systems generally require round, hemispherical radomes, which generate high drag, decreasing aerodynamic performance. Such platforms also typically locate the relatively soft optics for the platform in the central body region of the platform, leading to low lethality, consuming space otherwise available to a radar antenna, actuator, motor or thrusters.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.